Tetsuichi Takagi

Petrology of a mantle-derived rhyolite, Hokkaido, Japan

Abstract The Miocene Kitami rhyolite, consisting of orthopyroxene and plagioclase-phyric lavas and dikes, occurs on the back-arc side of the Kuril arc with coeval basalts and Fe-rich andesites. Temperatures estimated from orthopyroxene–ilmenite pairs exceed 900°C. Although the whole rock compositions of the Kitami rhyolite correspond to S-type granites (i.e., high K, Al, large ion lithophile elements, and low Ca and Sr), Sr–Nd isotope compositions are remarkably primitive, and similar to those of the coeval basalts and andesites. They are distinct from those of lower crustal metamorphic rocks exposed in the area. Comparison of chondrite-normalized rare earth element (REE) patterns between the rhyolite and the basalts and andesites show that the rhyolite is more light REE enriched, but has similar heavy REE contents than the basalts. All rhyolites show negative Eu anomalies. The geochemical data suggest that did not formed by simple dehydration melting of basaltic rocks or fractional crystallization of basaltic magmas. The features of slab-derived fluids expected from recent high pressure experimental studies indicates that mantle wedge is partly metasomatized with “rhyolitic” materials from subducted slabs; it is more likely that very low degree partial melting of the metasomatized mantle wedge formed the rhyolite magma.

Lithium and strontium isotopic systematics in playas in Nevada, USA: constraints on the origin of lithium

Lithium-rich brine in playas is a major raw material for lithium production. Recently, lithium isotopic ratios (δ7Li) have been identified as a tool for investigating water–rock interactions. Thus, to constrain the origin of lithium in playas by the use of its isotopes, we conducted leaching experiments on various lacustrine sediment and evaporite deposit samples collected from playas in Nevada, USA. We determined lithium and strontium isotopic ratios and contents and trace element contents of the leachate, estimated the initial δ7Li values in the water flowing into the playas, and examined the origin of lithium in playas by comparison with δ7Li values of the possible sources. In samples from the playas, δ7Li values show some variation, reflecting differences both in isotopic fractionation during mineral formation and in initial δ7Li value in water flowing into each playa. However, all δ7Li values in this study are much lower than those in river water and groundwater samples from around the world, but they are close to those of volcanic rocks. Considering the temperature dependence of lithium isotopic fractionation between solid and fluid, these results indicate that the lithium concentrated in playas in Nevada was supplied mainly through high-temperature water–rock interaction associated with local hydrothermal activity and not directly by low-temperature weathering of surface materials. This study, which is the first to report lithium isotopic compositions in playas, demonstrates that δ7Li may be a useful tracer for determining the origin of lithium and evaluating its accumulation processes in playas.

Fe-kaolinite in granite saprolite beneath sedimentary kaolin deposits: A mode of Fe substitution for Al in kaolinite

Abstract Fe-kaolinite has been detected in granite saprolite beneath sedimentary kaolin deposits in the Seto district of central Japan. Granite saprolite, which was found underneath sedimentary kaolin deposits formed in fluvial and lacustrine environments, had been subjected to kaolinization. The clay fractions of granite saprolite consist mostly of kaolinite with subordinate micaceous clay, quartz, and feldspars. Electron probe microanalysis (EPMA) showed that the kaolinite in clay fractions contained an average 3.30–3.72 wt% of Fe2O3, indicative of Fe-kaolinite. Fe+Si was inversely proportional to Al in Fe-kaolinite, indicating coupled substitution between Fe+Si and Al. The K2O contents of Fe-kaolinite increased with increasing Fe2O3 up to 0.77 wt%, whereas K did not correlate with other elements, suggesting that K was not contained with the structure of kaolinite but was present in its interlayers. X-ray absorption near-edge structure (XANES) spectroscopy showed that about 60 to 70% of Fe in the clay fractions is ferric iron, and extended X-ray absorption fine structure (EXAFS) spectroscopy indicated that Fe is situated in octahedral sites replacing Al. Fe-kaolinite was likely precipitated by the infiltration of acidic groundwater with higher Fe and alkali contents into granite saprolite, accompanied by the intense kaolinization of sedimentary kaolin deposits.